The present invention relates to a rubber composition for a tire and a pneumatic tire comprising the same.
In recent years, in order to improve properties that inhibit chipping of the bead toe of a tire (toe-chip resistance) and air permeability resistance, attempts are being made to improve the rubber composition used particularly for the chafer rubber or the inner liner among tire parts.
As a rubber composition for a chafer, elasticity is made high by using a rubber composition comprising natural rubber and butadiene rubber, in order to endure abrasion by the wheel rim flange and high load and prevent toe-chip (see JP-A-11-59143). However, chipping of the bead toe when attaching and detaching the rim and separation from the inner liner occur and there is the problem that air permeability resistance is poor.
Also, conventionally, a large amount of butadiene rubber (BR) is compounded in rubber compositions for chafers in order to suppress abrasion and heat generation. However, because BR tends to be deformed, a large amount of carbon black is added to harden the rubber and in such a case, chipping of the bead toe occurs when reassembling to the rim.
As a means to solve such problems, the method of using soft rubber only for the bead toe is suggested (see JP-A-7-81335), but according to this method, air permeability resistance cannot be improved. In order to improve air permeability resistance, the inner liner can be extended to the tip of the bead toe, but in such a case, separation occurs when assembling to the rim.
As shown in FIG. 3 which depicts a cross-sectional view of the bead of a pneumatic tire, the inner liner of a conventional pneumatic tire is usually positioned so that the end is located near bead core 6 or tucked beneath bead core 6. For bead toe 3, the same rubber is used as that used for the part that is worn out by the wheel rim flange (chafer rubber 2).
Consequently, in a structure as shown in FIG. 3 wherein the chafer rubber extends to the inside of the wheel, the air inside the tire cannot completely be sealed. When the inner liner is provided to the bead toe in order to seal in the air, problems such as separation and tearing of the inner liner occur when attaching and detaching the rim.
In this way, a rubber composition that can sufficiently improve toe-chip resistance and air permeability resistance of a tire has not yet been obtained.